Poppet valves are used in combustion engines to open and close intake and exhaust ports located in the engine cylinder head. These valves usually consist of a flat disk with a tapered edge rigidly connected to a long rod at one end, called a valve stem (shank). The valve stem is used to push down or pull up the valve against the tapered seat during opening and closing stages. A retaining spring is usually used to close the valve when the stem is not being pushed on. In conventional valvetrain system, the valve is raised from its seat by pushing the stem using a cam-follower mechanism. The cam profile and its location with respect to the cam follower determine the valve translational motion as well as its opening and closing timings. In the conventional designs, the camshaft is placed relatively close to the crankshaft and the translational motion from the cam follower is transferred to the valve stem through pushrods or rocker arms. This mechanism is very common in V-type engines and allows for the actuation of the valves of both cylinder banks using a common camshaft.
Conventional designs have considerable energy losses in the engine. The cams are usually fixed on the camshaft and rotate with the same speed as the camshaft. The camshaft obtains its rotary motion from the engine crankshaft using an intermediate mechanism such as chain, gear or belt. The camshaft speed is half the crankshaft speed in 4-stroke engines and equal to that in 2-stroke engines.
In addition to zero flexibility of the cam-follower valvetrains, another drawback of the cam-driven valvetrains is that the minimum possible engine valve opening angle (β) is limited due to the cam profile limitations. In a cam with flat faced follower, a negative radius of curvature on the cam cannot be accommodated and this limits the minimum cam rise or fall angle (β/2) for a specific cam size.
Significant improvement in power density, volumetric efficiency, emission and fuel consumption can be achieved by variable valve actuation systems (VA).
In general, VA systems are divided into two main categories: camless and cam-based valvetrains. In the camless systems, there is no mechanical connection between the engine crankshaft and the valvetrain. High level of flexibility in valve timing and valve lift is the main advantage of these systems over cam-based valvetrains. Electro-mechanical, electrohydraulic and electro-pneumatic valvetrains are all in this category. Although these systems are the most flexible valve actuation systems, some concerns including high cost, low reliability (i.e. not being fail-safe), high power consumption (>2.2 kW for 16 valve engine at 5000 rpm engine speed), high seating velocity (>100 mm·s−1) and control complexity (requires ultra fast actuator with response time of less than 3 ms) prevent these systems from being incorporated into production engines.
In contrast to camless valvetrains, the cam-based VA systems are mechanically linked to the engine crankshaft. Due to their high reliability, durability, repeatability and robustness, many of these systems have been already designed and implemented in production engines. Limited flexibility and high mechanism complexity are the major disadvantage 5 of the cam-based valvetrains compared with the existing camless systems.
Cam phaser is a standard mechanism for valve timing. By using this mechanism, it is possible to change the cam angular position relative to the crankshaft and consequently shift the valve opening and closing events simultaneously. However, using this mechanism, the total engine valve opening duration and lift remain constant. Cam phasers are categorized into oil actuated, helical gear drives, differential drives, chain drives, worm gear drives and planetary gear drives.
Cam Profile Switching (CPS) is another technique introduced by Honda to vary valve timing, duration and lift simultaneously. In this technique, the valve motion is switched between two different sets of cam lobs. During low engine speed operation, the cam with low lift profile is engaged with the valve stem, while at high engine speed operation, the cam with high lift profile is engaged. The shift from one cam to another is realized by either an electric or hydraulic system. In this system, the cam profiles are compromised settings for the desired objectives during two engine speed ranges.
One of the problems of cam profile switching is that the valve motion is switched only between two specific cam profiles. However, the use of a three-dimensional cam design allows the engine to continuously change the valve timing, lift and duration over a wide range of engine operating conditions. In this mechanism, the cam profile continuously varies along the cam axis, and the axial movement of the camshaft with respect to follower brings a different profile of the cam into engagement with the follower, prompting a change in valve opening profile. A combination of three-dimensional cam mechanism and cam phaser has been also implemented by Nagaya et al. to control both valve timing and valve lift independently.
Electromagnetic valve actuation systems generally consist of two magnets and two balanced springs. The moving parts of the electromagnetic valve are connected to the engine valve. When both magnets are off, the armature is held in the intermediate position between the coils by balanced springs.
At engine start-up, the upper electromagnet is activated and it pulls up and holds the armature, and the potential energy is stored in the retaining springs. To open the valve, the upper electromagnet is deactivated and the stored energy is released and converted into kinetic energy which carries the armature toward the lower magnet. At a distance of less than one millimeter from the lower magnet, the moving part is captured and held. During the valve closing stage, similar events are repeated. Due to high non-linearity in magnetic force characteristics, there are several difficulties preventing this technology. from being commercially implemented [30]. These difficulties include: High landing velocity (>0.5 msec at 1500 rpm), High transition time (>3.5 msec), Higher power losses than conventional cam drive system, Requirements for robust feedback control, High sensitivity to in-cylinder gas pressure.
A basic electro-hydraulic camless valvetrain consists of a hydraulic cylinder, two solenoid valves and two check valves. In this design, the solenoids and the check valves control the submission and rejection of the high pressure oil into and out of the hydraulic cylinder during valve operation. Using an additional oil path, a constant force is always applied to the bottom of the piston, and when the high pressure oil is removed from the piston top, the valve returns to its seated position. By controlling the solenoid valves timing and opening duration, it is possible to precisely control the valve timing, duration, and lift. By activating the high pressure solenoid valve, the high pressure oil is admitted into the hydraulic cylinder. The opening period of this high pressure solenoid valve determines the amount of oil submitted into the cylinder chamber and consequently determines the valve lift. By activating the low pressure solenoid valve, the oil is discharged from the upper cylinder chamber thanks to the presence of high pressure oil at the lower chamber. The low pressure solenoid valve opening duration determines how far the valve moves in its closing descent.
Similar to electro-mechanical valve systems, a closed loop electronic control is required to reduce valve seating velocity, transition time, and cyclic variability. One of the problems of this VVT system is servo-valve response time. Due to solenoid coil inductance and nonlinear force to displacement relation, the solenoid maximum operating frequency is reduced and, as a result, the system shows poor performance during high engine speeds.
The required valve actuation time reduces significantly as engine speed increases, and consequently the minimum valve opening angle becomes limited. For example, at an engine speed of 6000 rpm and a total opening angle of 100 degrees, the total time available for the actuation process is about 3 ms, which almost exceeds the speed of the high bandwidth solenoid valves which are currently on the market. This causes the electrohydraulic valve manufacturers to use either a double-stage mechanism (i.e., two pilot valves) or employ ultra high frequency actuators such as piezoelectric.
In electro-hydraulic VA systems, the major part of the system cost is for high speed servo-valves which control the oil flow to and from the hydraulic cylinder. A high speed servo valve may be split into a digital three-way valve and two proportional valves. The digital three-way valve directs hydraulic fluid either from a high pressure source toward the hydraulic cylinder or from the hydraulic cylinder to the reservoir. However, the two-way proportional valves control the valve timing, valve rise/fall duration, final valve lift and valve velocity.
An electro-hydraulic valvetrain has been proposed by Brader et. al in which the solenoid actuators are replaced with piezoelectric stacks. The proposed system is capable of having maximum valve lift of 12.4 mm and bandwidth frequency of up to 500 Hz. In this mechanism, an electric signal sent from a control system causes a piezoelectric stack to expand. This linear expansion is transferred to the spool valve via a solid hinge mechanism. The reason for using this mechanism is to overcome the displacement limitations in the piezoelectric stacks while maintaining its efficiency and operating frequency. Using this mechanism, the movements of the stacks can be amplified from 30 μm to 150 82 m, which is sufficient for spool valve actuation.
In addition to electro-hydraulic and electro-mechanical valvetrains, electro-pneumatic variable valve actuation systems are proposed. The combination of hydraulic and pneumatic mechanisms allows the system to extract maximum work from the air flow and thus it can function under low air pressure. To reduce the energy consumption and also control valve seating velocity, a hydraulic latch was also employed in this system. This mechanism is capable of controlling valve lift, valve timing, and opening duration as desired by the engine.
One of the main problems of this system is its high dependency on the in-cylinder gas pressure. Due to low working pressure of this system compared to hydraulic systems and gas compressibility, the valve opening and closing are highly affected by the engine in-cylinder pressure. Thus, having pre-knowledge of the cylinder pressure and also solenoid response time is necessary to predict the exact timing of solenoid activation or deactivation. The solenoids response time also limits the system's bandwidth.